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SM73302_15 Datasheet, PDF (23/31 Pages) Texas Instruments – SM73302 88 MHz, Precision, Low Noise, 1.8V CMOS Input, Decompensated Operational Amplifier
◁
SM73302
www.ti.com
SNOSB93A – AUGUST 2011 – REVISED APRIL 2013
The expression fGBW is the gain bandwidth product of the part. For a unity gain stable part this is the frequency
where A = 1. For the SM73302 fGBW = 88 MHz. Multiplying A and F results in the following equation:
AF
´
¶P
=
´
¶ GBW
´
¶
x
1 + sCFRF
=
1 + sRF (CF + CIN)
´
¶ GBW x
´
¶
1+
§
¨
CFRF
¨
©
CFRF
2
=1
1+
§
¨
¨
©
RF (CF + CIN)
CFRF
2
(16)
For the above equation s = jω. To find the actual amplitude of the equation the square root of the square of the
real and imaginary parts are calculated. At the intersection of F and A, we have:
Z= 1
CFRF
(17)
After a bit of algebraic manipulation the above equation reduces to:
1+
§
¨
CF
+
CIN
¨
©
CF
2
= 8S2 ´¶G2BW RF2 CF2
(18)
In the above equation the only unknown is CF. In trying to solve this equation the fourth power of CF must be
dealt with. An excel spread sheet with this equation can be used and all the known values entered. Then through
iteration, the value of CF when both sides are equal will be found. That is the correct value for CF and of course
the closest standard value is used for CF.
Before moving to the lab, the transfer function of the transimpedance amplifier must be found and the units must
be in Ohms.
VOUT =
-RF
1 + sCFRF
x
IDIODE
(19)
The SM73302 was evaluated for RF = 10 kΩ and 100 kΩ, representing a somewhat lower gain configuration and
with the 100 kΩ feedback resistor a fairly high gain configuration. The RF = 10 kΩ is covered first. Looking at the
Input Common Mode Capacitance vs. VCM chart for CCM for the operating point selected CCM = 15 pF. Note that
for split supplies VCM = 2.5V, CIN = 22 pF and fGBW = 88 MHz. Solving for CF the calculated value is 1.75 pF, so
1.8 pF is selected for use. Checking the frequency of the pole finds that it is at 8.8 MHz, which is right at the
minimum gain recommended for this part. Some over compensation was necessary for stability and the final
selected value for CF is 2.7 pF. This moves the pole to 5.9 MHz. Figure 61 and Figure 62 show the rise and fall
times obtained in the lab with a 1V output swing. The laser diode was difficult to drive due to thermal effects
making the starting and ending point of the pulse quite different, therefore the two separate scope pictures.
Figure 61. Fall Time
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